Hybrid refractive/diffractive achromatic camera lens and camera using such

ABSTRACT

A hybrid refractive and diffractive achromatic lens suited for single-use or inexpensive single-lens cameras operating in visible spectral range. The lens comprises a body of optically transmissive material having an index of refraction at a wavelength approximately at the center of said range of at least 1.45. The lens body having a first and second surface on opposite sides thereof, at least one of said surfaces being curved to provide a refractive portion having power and introduces chromatic aberration, the lens having a diffractive portion having power which substantially achromatizes the lens for the chromatic aberration of the refraction portion over about 440 nm to about 650 nm range.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation-In-Part of application Ser. No.175,708, filed on Dec. 29, 1993, in the name of Mark M. Meyers now U.S.Pat. No. 5,543,966.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to hybrid refractive/diffractive lensesand particularly to a refractive/diffractive achromat which isespecially suitable for use as a taking (or an objective) lens ininexpensive cameras such as single-use cameras.

2. Description Relative to Prior Art

In order to obtain photographs with good quality images, the lens thatfocuses the light must be well corrected for aberrations. It is notenough for the objective lens to be corrected for monochromaticaberrations. The lens must also be corrected for chromatic aberrationsfor a relatively broad range of wavelengths. For each color orwavelength of light incident on a refractive lens, the lens will have adifferent focal length. It is this property of the lens that give riseto (longitudinal and lateral) chromatic aberrations. Currently, theobjective lenses for cameras correct chromatic aberrations by usingadditional lens elements. However, this creates additional bulk andmakes the lens system heavier and more expensive. These considerationsare especially important for single use cameras which need to be lightweight, compact and inexpensive.

Single-use cameras typically include a one or two element lens utilizedat a large F/# so they can be used In a fixed focus mode whereeverything from two meters to infinity is nearly in focus. Single-usecameras of a single lens element type typically are not corrected forchromatic aberrations, which all singlets tend to suffer from. Lensesused for single-use cameras generally have relatively high levels ofmonochromatic and chromatic aberrations. Some of the monochromaticaberrations can be corrected in a plastic molded singlet element throughthe use of aspheric surfaces. However, at some point, the chromaticaberrations will be significantly worse than the monochromaticaberrations therefore limiting the minimum spot size. The resultingunachromatized images can also exhibit color fringing.

Current single-component objective lenses used in single-use cameras aremade of low dispersion, low index of refraction materials (usuallyplastic) to minimize longitudinal chromatic aberration. Thus, in orderto reduce the difficulty of correcting for chromatic aberration in asingle-element lens system, lens designs have been driven in thedirection of reducing dispersion (using low index, high Abbe numberglass) in order to obtain the necessary power and reduce the numericalaperture (NA) of the lens. Higher curvature, thicker lenses havetherefore been required. Such thicker lenses give rise to manufacturingerrors since they are more sensitive to variations in lens thickness,wedge, tilt, and decentering.

Additional lens elements are used to provide chromatic aberrationcorrection in multi-element, more costly, lens systems. When a cementeddoublet (comprised of a positive and a negative power lens element) isused to correct for chromatic aberrations, a negative power lens elementmade of flint glass (i.e. glass having a low Abbe number) is cemented tothe positive lens element which is typically made from a crown glass,However, because the negative lens element increases the focal length,the positive lens element is made stronger to compensate for that changein order to keep the original focal length. In order to obtain thenecessary power, the positive lens element will thus need to havestronger radii of curvature and to be thicker. Such lenses alsosacrifice weight and size in order to accommodate surfaces and elementswhich compensate for chromatic aberration. Alternatively, two airspaced, roughly symmetrical, lens elements separated by an aperture stopcan also be used to get a better system performance. However, anadditional element again increases weight and size of the system.Finally, when designing a single element optical system, a designer mayuse low dispersion glasses that still have a high index of refraction.However, such glasses are expensive.

Although various patents and publications have discussed the use ofdiffractive elements to compensate for chromatic aberration (see U.S.Pat. Nos. 4,768,183, 5,117,433, 5,044,706, 5,078,513, 5,117,306,5,161,057, 5,208,701, and 5,229,880), designs for objective or takinglenses in single element cameras have not had any chromatic correctionand typically have relatively steep surface curvatures. As previouslymentioned, in order to avoid these and other problems, some single-usecamera lens systems include two lens elements separated by an aperturestop. Similarly, consumer camera lenses in visible light applications,such as for taking photographs of friends, relatives or nature, usemultiple lens elements to correct for chromatic aberrations.

SUMMARY OF THE INVENTION

The present invention deviates from the conventional wisdom in the fieldof optical design for camera lenses (such as optical objectives)operating in visible spectrum by modifying one of the surfaces of theplastic lens (or a glass lens) in a simple camera objective such as theone used in a single-use camera to achromatize the lens in the visiblespectrum with a diffractive surface. This invention is especially usefulin simple cameras using lenses with F-numbers of f/5.6 and higher andespecially those lenses of f/8-f/12 range and even more preferably f/9to f/11 range. This is because this lens needs to be produced relativelyinexpensively, should not be bulky, needs to have as few elements aspossible and yet is able to produce good pictures where the object beingphotographed may be far away or may be standing as close as two metersaway from the photographer. By using a correctly designed diffractivesurface the level of aberrations can be reduced by a factor of two ormore and the lens can be achromatized over the entire visible spectrumeven for a single element. Diffractive surfaces can also incorporateaspheric terms at low cost, thereby, obtaining a thinner, lighter lens.It has been discovered in accordance with the invention that thechromatic aberration (of the light in at least 150 nm range) introducedby a refractive portion of the lens can be corrected by a diffractiveportion which is incorporated into the lens thereby providing animproved hybrid refractive/diffractive achromatic lens for an objectivecamera lens application such as a single-use camera.

The invention also provides an improved taking or objective lens such asone for use in a single-use camera and has one or more of the followingaspects: (a) high polychromatic MTF such as MTF value of about 0.4 orhigher at a broad range of frequencies; (b) improved chromaticcorrection in visible spectrum; (c) a singlet rather than amulti-element configuration; and (d) small size and light weight.

Objective camera lenses, including single lens camera lenses, have tofocus light in a band-width of almost a 300 nm wavelength range (e.g.,from 400 nm and above to 680 nm (with the emphasis on a center portionof this region). A hybrid refractive/diffractive achromat for use incameras in accordance with the invention comprises a body of opticallytransmissive material having an index of refraction at a wavelengthapproximately in the center of said range of at least 1.45, said bodyhaving first and second surfaces on opposite sides thereof, said firstand second surfaces being intersected successively by an optical axis ofsaid lens which extends in a longitudinal direction, at least one ofsaid surfaces being curved to provide a refractive portion havingoptical power and introducing longitudinal chromatic aberration, saidlens having a diffractive portion having optical power whichsubstantially achromatizes said lens for said chromatic aberration ofsaid refractive portion over at least 490 to 650 nm or at least 440 to625 nm region.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will become more apparent from a reading of the followingdescription in connection with the accompanying drawings in which:

FIG. 1 is a schematic diagram of a refractive/diffractive hybrid lens 10in accordance with the invention shown spaced from photographic filmupon which an image is formed, the lens having a curved surface 1 and adiffractive grating surface 3 formed on curved surface 2, the featureson the diffractive surface being too small to be seen on the scale ofthe Figure;

FIG. 2 is a sectional view of the lens shown in FIG. 1 but thediffractive surface features are greatly magnified;

FIG. 3 is a diagrammatic, perspective view of a Fresnel zone patternwhich may be formed as by blazing on the diffractive surface of the lensshown in FIGS. 1 and 2, where λ₀ is the design wavelength, m is aninteger greater than 0, f is the focal length and F designates the focalpoint;

FIG. 4 is a greatly enlarged side view of the diffractive surface of thelens shown in FIGS. 1 and 2 showing the surface blaze profile of a fewzones, the actual thickness or height h of each zone being of the orderof 0.8-1.4 μm and the spacing between the zones actually being of theorder tens of microns (40 μm-600 μm);

FIG. 4A is an enlarged vie of a single blaze step of the side view ofFIG. 4;

FIG. 4B is an enlarged vie of a portion of the blaze surface of thesingle blaze step of FIG. 4A;

FIG. 5 is a plot of the efficiency of the lens for design wavelength λ₀of 587.6 nm;

FIG. 5A is a schematic diagram of a prior art lens having a binary-typediffractive surface;

FIGS. 6A and 6B are plots of the ray aberration of the refractivediffractive lens of FIG. 1 for on axis, 0.7 field as well as full field.The full field is ±32°. FIG. 6A shows the ray aberration correspondingto the vertical axis of 0.40 mm (±0.20 mm) while FIG. 6B shows similarcurves where the vertical axis is one wavelength (± 1/2λ);

FIG. 7 is a plot of the on axis, polychromatic MTF of therefractive/diffractive lens of FIG. 1;

FIG. 8 is a plot of an on axis, polychromatic point spread function ofthe refractive/diffractive hybrid lens of FIG. 1;

FIG. 9 is a plot of the optical path difference in the image planegenerated by the refractive/diffractive lens of FIG. 1;

FIG. 10 is a plot of the polychromatic MTF corresponding to the 0.7field, i.e., a half field angle of 22.4° of the refractive/diffractivehybrid lens of FIG. 1;

FIG. 11 is a plot of a polychromatic point spread function correspondingto the 0.7 field, i.e., a half field angle of 22.4° of therefractive/diffractive hybrid lens of FIG. 1;

FIG. 12 is a plot of the polychromatic MTF corresponding to the fullfield, i.e., a half angle of 32° of the hybrid lens of FIG. 1;

FIG. 13 is a plot of a polychromatic point spread function correspondingto the half field angle of 32° of the hybrid lens of FIG. 1;

FIG. 14 is a schematic diagram of a second embodiment of arefractive/diffractive hybrid lens made in accordance with the inventionand shown forming an image on a photographic film, the lens having acurved surface 1' and a diffractive grating surface 3' formed on curvedsubstrate surface 2', the features on the diffractive surface being toosmall to be seen on the scale of the Figure;

FIG. 15 is a graph of a diffraction efficiency profile of the lenssystem of FIG. 1 where the diffractive profile is optimized for λ=0.540μm; to FIG. 16 is a graph of a diffraction efficiency profile of thelens 20 shown in FIG. 14 where the diffractive profile is optimized forλ=0.510 μm;

FIG. 17 is a schematic drawing of the manufacturing apparatus used toproduce diffractive surfaces 3 and 3' of refractive/diffractive hybridlenses shown in FIGS. 1 and 14;

FIG. 18 shows a sharp diamond tip with a small flat used in an apparatusof FIG. 17;

FIG. 19 shows a prior art rounded diamond tip;

FIG. 20 shows a sharp diamond tip with no flat used in an apparatus ofFIG. 17;

FIG. 21 shows that a diamond tip of FIG. 18 is canted at an anglerelative to the work piece;

FIG. 22 shows final surface roughness characteristics of the preferredembodiment as seen through the atomic force microscope;

FIGS. 23A-D are plots of polychromatic MTF curves corresponding to 0,0.5, 0.7 and full field of view of lens 20 of FIG. 14;

FIG. 24 is a schematic drawing showing how lens 10 or 20 may be molded;

FIG. 25 is a schematic diagram of a third embodiment of arefractive/diffractive hybrid lens made in accordance with the inventionand shown forming an image on a photographic film, the lens having acurved surface 1" and a diffractive grating surface 3" formed on curvedsubstrate surface 2", the features on the diffractive surface being toosmall to be seen on the scale of the Figure; and

FIGS. 26A-F are respectively plots of polychromatic MTF curvescorresponding to 0, 0.4, 0.6, 0.75, 0.9 and full field of the lens 30 ofFIG. 25.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, there is shown a diffractive/refractive hybrid lens10 for use in visible light camera applications and more specificallyfor use in a single-use camera 100 having a camera body 5 and aphotoghraphic film 15 supported at a suitable location so that an imageis produced by the lens on the film. The lens 10 is a convex-concavesingle element or singlet having from an object side a convex-curvedsurface 1 and a Fresnel zone pattern 3 on the concave-curved surface 2of the lens body which is the surface opposite to the first curvedsurface 1. Both surfaces 1 and 2 are perpendicular to the optical axisof the lens. The refractive lens is made from optically transmissivematerial having an index of refraction of at least 1.49. Suitable andpreferable material is any moldable material such as optical plastic orglass. If the lens element will be molded, a plastic material is morepreferable because it is cheaper and easier to mold. Alternatively, ifthe lens is not to be molded, a diffractive zone pattern may be diamondturned or cut on the lens surface. If the lens is not to be molded, thelens material does not need to be suitable for molding purposes. FIG. 2shows the lens 10 and emphasizes its curved surface 2 which defines therefractive portion of the element as well as the Fresnel zone pattern 3which defines the diffractive portion of the element. An annular ring 16is part of the lens body and is merely for attachment and location in acamera barrel. The lens F-number is f/11 and the lens accommodates afield angle of ±32 degrees. The overall thickness of the lens may beless than 4 mm and it is preferred that it be about 1.0-3.0 mm(millimeters). It is 1.4 mm in this embodiment. The focal length of lens10 is 35 mm and it accommodates the field angle of ±32°. The base radiusof the curved surface (or substrate) 2 on which the diffractive surface3 of the lens 10 is formed (shown in FIG. 3), is located is 11.43 mm toa point along the optical axis on the right of that surface. Thediffractive surface 3 of the diffractive portion of the lens element 10has a radius of curvature that corresponds to an effective focal lengthf of 364.2 mm. Exemplary dimensions and spacings are set forth in Table1, the index n.sub.λn n is at the center of range being measured at thenominal wavelength λ_(n) =587 nm (or 0.587 μm) and n.sub.λn is 1.496.

                  TABLE 1                                                         ______________________________________                                        Sur-                                    V                                     face  Radius    Thickness Material                                                                              Index Number                                ______________________________________                                        1     7.42      1.4       Plexiglass                                                                            1.492 57.3                                  2     11.4343.sup.1                                                                           0         Plexiglass                                          3     NA.sup.2  0                 10,000                                                                              -3.5                                  Air             4.309     Air     1.    1.                                    Stop            28.3096                                                       Image -120.000.sup.3                                                          plane                                                                         ______________________________________                                         1 Base radius is 11.4343. This surface is an "internal" surface used for      design purposes, i.e. there is no index difference between surfaces 2 and     3 in this embodiment.                                                         2 This surface profile is an asphere; radius of curvature corresponds to      focal length of = 364.2. The aspheric profile of the surface as described     by equation 12; and where AD = 0.8265207E 8; AE = -0.1041272E 8; AF =         0.612808E 10; and AG = -.1356105E11.                                          3 Cylindrical shape.                                                     

The lens of the first embodiment has a nominal or center wavelength ofλ_(n) =λ_(d) =587 nm. The lens is achromatized for 480 to 680 nmbandwidth around this center wavelength. Specifically, the designwavelengths are: λ_(f) =486 nm, λ_(d) =587 nm, and λ_(c) =656 nm.

The depth h of echelons in the zones is shown exaggerated in FIGS. 2 and4 and may be of the order of a 0.8 to 1.4 micron and it is preferablethat they be 0.9 to 1.2 microns. The spacing d between the zones in thisembodiment is between 40 and 600 microns (μm). It is preferable that thespacing d be on the order of tens of microns, but it can be 1 to 1000μm.

It is preferable that the diffractive surface 3 be formed on the curved(base) surface such as surface 2 which acts as a substrate. In thisembodiment, the diffractive surface is formed on a concave surface withradius of curvature of 11.43 mm. However, the concave surface 2 in thisembodiment is an internal surface of the lens and is not a real,separate surface because the same index material is used to mold theentire lens.

The achieved achromat is a single element or singlet, but in effectworks as a cemented doublet because the refractive and diffractiveportions of the lens element work together to add to the final totalpower. It is preferred that the refractive portion of the lens componenthas 85-97% of total power of the lens component. Table 1, which will bediscussed in greater detail below, shows that about 90% of the totalpower of the achromat is in the refractive portion of the lens elementand about 10% is in the diffractive portion of the lens element.Therefore, the achromat behaves much like a regular singlet. Since bothsurfaces of the refractive portion of the lens element are curved (i.e.the front surface 1 and the internal or substrate surface 2), there areat least two degrees of freedom to modify the lens in order to controlaberrations. In addition, one or both of the real or actual surfaces(surfaces 1 and/or 3--i.e. external surfaces) may be aspheric. In thisembodiment, aspheric terms on the diffractive surface allow for betteraberration correction. The diffractive surface compensates forlongitudinal chromatic aberration, but also because of introduction ofhigher order terms (4th order, 6th order, 8th order and 10th ordercorresponding to AD, AE, AF and AG coefficients), in the phase function[equation (4)] of the diffractive surface 3, monochromatic aberrations,such as spherical aberration and coma are also substantially corrected.

Consider the design of the diffractive surface 3. The design takesadvantages of the wave nature of light. Light travels in waves, whichcan interfere. If the waves interfere such that the peaks and valleyscoincide, the energy in the two waves adds to each other; this isreferred to as constructive interference. Note that if one of the wavesis delayed exactly one or more wavelengths behind the other, then it isonce again in phase, and they will interfere constructively. If thewaves line up out of phase, the energy in one wave will cancel theenergy in the other; this is referred to as destructive interference.

To design the diffractive surface, a diffractive zone pattern is used,as shown in FIGS. 3 and 4. Such a zone pattern consists of multiplezones Z_(i). A focal point, F, is designated at a distance, f, from thecenter of the pattern. This distance is equal to the focal length. Therings, or zones, are spaced such that the edge of each zone is exactlyone wavelength further away from the point F. This way light passingthrough the pattern at the edges of the zones will be in phase andconstructively interfere at the point F.

Using right triangles, an equation can be derived that gives the zoneradius or zone spacing r_(m) as a function of the focal length f(distance from the pattern to F) and the wavelength of light λ₀ used todesign the zone pattern (i.e. blaze wavelength) and m is a zone number:

    r.sup.2 +f.sup.2 =(f+mλ.sub.o).sup.2.               (1)

Assuming the wavelength of light is much smaller than the focal length,Equation (1) can be reduced to: ##EQU1##

From Equation (3), it can be seen that the diffractive surface has astrong dependence on the wavelength of light used to construct thezones. If the wavelength of light incident on the diffractive surfacedeviates from the design wavelength, the focal length also changes. Thisis an important property when the diffractive surface is used toachromatize the refractive element.

Although the light propagating from the edge of zone is in phase when itgets to the focal point F (FIG. 3), light coming through the middle ofeach of the zones is not yet in phase, and therefore will not interfereconstructively. To correct this problem, material is taken off (i.e. itis machined off if the diffractive surface is diamond turned) in aprogrammed manner in accordance with a profile desired so that the phaseis delayed just enough so that at the point F, all the light comingthrough the surface constructively interferes. This blaze is shown inFIG. 4. Wherein, it can be seen that a step or zone is tapered towardsthe substrate. The tool is brought in to remove the programmed amount ofmaterial and then is brought out as the surface makes a spiral-like cutin the surface.

In the center of the first zone Z₀, where the material is the thickest,the light is delayed exactly one wavelength. Moving away from the centerof the pattern, the distance from the focal point increases so that lessmaterial is needed at the periphery of zone Z₀. The material isgradually thinned to a minimum at the edge of the first zone, where noadditional delay is needed, because the distance at the edge of thefirst zone is one wavelength further from the focal point than thecenter of the ring pattern. Again material is added at the next zone Z₁to delay the light exactly one wavelength, but the light is still inphase. Since the material is once again thick, the process starts over.This way all the light passing through the diffractive surface will bein phase and constructively interfere at the focal point. In general,the phase delay introduced by such a surface can be described withEquation (4): ##EQU2##

Giving value to the higher order phase terms, A₄, A₆, etc., has the sameadvantage as introducing aspherical terms (AD, AE, AF and AG) for aglass (or plastic) surface, and is useful for minimizing monochromicaberrations. Whenever φ is equal to an integer multiple of 2π, r is theradius of a new zone.

Using Equation (4), it is possible to design a diffractive surface thatis nearly 100% efficient at the desired focal point. The efficiency ηchanges, however, with change in the wavelength of incident light, in amanner described by: ##EQU3## where λ_(o) is the design or blazewavelength and λ is the incident wavelength. For a design λ_(o) or blazewavelength λ_(o) =λ_(b) =0.587 μm (i.e., 587 nm) used in the firstembodiment (which is the same as the nominal or center wavelength λ_(n)=λ_(d)), the efficiency as a function of wavelength is plotted in FIG.5. However, an objective camera lens should work well for light at awavelength range of at least 486 nm to 656 nm or about 490 nm to 650 nm.The undiffracted light, i.e. light not diffracted to the right focalpoint, becomes unwanted stray light at the focal plane. It creates ahalo-like or a smear-like effect and decreases the lens efficiency.

The lens shown in FIG. 1 is made utilizing a "linear blaze" methodrather than a "step function" blaze method. A "step function" or"binary" kind of blazing typically results in 4 to 16 discrete stepscomprising each zone (FIG. 5A). The examples of such surfaces areprovided in U.S. Pat. No. 4,768,183 and U.S. Pat. No. 5,044,706. Thediffractive surface of the lens system of this embodiment does not havezones made out of four to sixteen discrete steps and is found to have amuch higher efficiency at the nominal or design wavelength (λ_(n) =λ_(d)=λ_(o)). While a lens with a "step" blaze would have an efficiency ofabout 95% (an eight step zone profile resulting from a three maskprocess) or less at the normal wavelength, this lens has an efficiencyof about 100% at the nominal wavelength of 587 nm. The "Linear Blaze"method of manufacturing will be described later in the specification.

By equating Equations (6a) and (6b), for the powers of a thin glass lensφ_(th) and a diffractive lens φ_(diff), Equation (7) is obtained.Equation (7) is used to find the index of refraction with light atwavelengths other than the design wavelength λ_(o). ##EQU4##

One measure of how dispersive glasses are, that is, how much their indexchanges with change in wavelength, is the Abbe ν-number formula,Equation (8). Three wavelengths are picked, and their indices aresubstituted into Equation (7): ##EQU5## where n_(f), n_(d) and n_(c) arethe indices of refraction for the short, middle, and long wavelengths.When evaluating optical materials, the wavelengths chosen are usuallyλ_(f) =0.48613 μm, λ_(d) =0.58756 μm, and λ_(c) =0.65627 μm. For allglasses, the ν number for these three wavelengths is between 20 and 90.Typically, the lower the ν number, the further n_(f) and n_(c) are fromeach other, and the glass is more dispersive. However, somewhatdifferent wavelength range may be used for achromatizing the lens. Inaddition, the proper choice of a nominal wavelength may increase theoverall diffraction efficiency of the lens. This is discussed in detailin conjunction with the second lens embodiment.

If Equation (7) is substituted into Equation (8), the Abbe ν number fora diffractive lens is found to be ##EQU6## If λ_(f) =0.480 μm, λ_(d)=0.5876 μm, and λ_(c) =0.656 μm are chosen for short, long and centerwavelengths respectively, as was done in the first embodiment, thenusing Equation (9), the ν number for a diffractive lens in the visiblerange for 0.496 μm<λ<0.656 μm is found to be

    ν.sub.diff =-3.5.                                       (10)

This number has very important ramifications when achromatizing lenses.As was mentioned previously, one may want to achromatize the lens forthe entire 0.4 μm to 0.68 μm or from 0.47 to 0.67 μm, or at least fromabout 0.49 μm to about 0.65 μm.

The power of the refractive element is balanced with the power of thediffractive surface. The power of each element adds to equal the desiredpower of the achromat φ_(tot), and the powers are also of the correctproportion so that the longitudinal chromatic aberration is zero. Thesepowers are given by: ##EQU7## where ν_(ref) and ν_(diff) are the νnumbers for the refractive and diffracted elements, φ_(ref) and φ_(diff)are the powers of the two elements, and φ_(tot) is the total power ofthe lens. One may use either a low dispersion material (glass) or a highdispersion material for use in a refractive element. The trade-off is asfollows: the focal length of the entire lens φ_(tot) tot is heldconstant. As the ν_(ref) becomes smaller (i.e., the material becomesmore dispersive) the index or refraction n_(ref) will typically raise.The higher is the index of refraction of the refractive system, thesmaller (i.e. shallower) is the resulting Petzval curvature of theoptical system. However, as ν_(ref) gets smaller, one will need morediffractive power.

Once the required focal length for the diffractive surface is found,zone spacings which results in this focal length are then determined asdiscussed in connection with FIGS. 3 and 4. Once the proper zonelocations are determined, they are then fine-tuned to minimize field(monochromatic) aberrations, as discussed in connection with Table 1.

Equation (10) shows the ν number for a diffractive lens is ν_(d) =-3.5.Since the lowest ν number for glasses is about 20, the diffractive lensis found to be much more dispersive than any refractive lens. It is alsoseen to be negative, where all glasses are positive. Equations (10a) and(10b) can thus be used to design a diffractive/refractive hybridachromat with a focal length of 35 mm. Typical values for this lens areshown in Table 2 (typical values for a 35 mm focal length).

    ______________________________________                                                                           % of total                                                                    power                                      Portion n.sub.d  ν number                                                                            power φ                                                                            f (=38.7 mm)                               ______________________________________                                        Moldable                                                                              1.492    57.3     2.58 × 10.sup.-2                                                                 90.3%                                      acrylic                                                                       Diffractive                                                                           10,0001* -3.5     2.75 × 10.sup.-3                                                                 9.7%                                       ______________________________________                                         *This is a theoretical number used for design purposes.                  

Because of the unusually low, and negative, ν number for the diffractivesurface, Table 1 shows a weak positive diffractive portion of the lenselement can be used to achromatize the lens. Since the powers of the twolens portions are no longer working against each other (i.e. they do nothave opposite signs), the need for the power in the refractive portionof the lens is further reduced by using the diffractive portion of thelens. The advantage of having less power in the refractive portion ofthe lens element is a thinner lens with lower surface curvatures andreduced aberrations. Because the refractive portion of the lens elementis thinner and because there is no bulky negative element, thediffractive/refractive hybrid achromat is much lighter than a solelyrefractive achromat made from glass or plastic. Additionally, becausethe surface curvatures (FIG. 1) are not steep, the refractive portion ofthe lens element in the hybrid introduces lower monochromaticaberrations than would be present in an all glass achromat.

The aspheric coefficients for the diffractive surface 3 are defined byEquation (12): ##EQU8## where z is the surface sag for the diffractivesurface from a x-y plane tangent to the surface, C is the surfacecurvature, AD, AE, AF, AG, are the fourth, sixth, etc. asphericcoefficients, and r is a radial coordinate in the lens (shown in FIG.3). An example of suitable coefficients is given in Table 1. Theaspheric coefficients relate to the phase coefficients for thediffractive surface, defined in Equation (4). The wavefront coefficientsare related to the surface sag coefficients by:

    CVW=C*(n'.sub.c -1)

    KW=K*(n'.sub.c -1)

    D=AD*(n'.sub.c -1)

    E=AE*(n'.sub.c -1)

    F=AF*(n'.sub.c -1)

    G=AG*(n'.sub.c -1)                                         (13)

where CVW=C*10,000 and n'_(c) ="construction" refractive index of thediffractive element which is set to 10,000 for high accuracy modeling).This index of 10,000 is not an actual refractive index of thediffractive element. CV is the surface curvature.

FIGS. 6A and 6B provide aberration curves for the refractive/diffractivelens. They show the performance of the lens for on axis, 0.7 field aswell as to the 1.0 field. The full field is ±32°. These figures showthat the refractive/diffractive lens, indeed, has a superior performance(a factor of 2 or better than a typical unachromatized lens element).For example, none of the aberration curves (in meridianal plane) exceed±0.02 mm (i.e. Δ<0.04 mm or a 40 μ.sub.λ, where Δ is a diameter of ablur circle).

FIG. 6B is a plot of the optical path difference (OPD) at 0.7 and 1.0field. It can be seen that OPD is less than ±0.5 wavelength throughoutthe field and thus the lens is nearly diffraction limited. The OPD onlyslightly exceeds it at 0.5 field at the edge of the aperture.

FIGS. 6A-6B shows that the lens design has a good performance out to thefull field, thus giving a wide margin of error for manufacturingtolerances.

The lens MTF plots are provided in FIG. 7, FIG. 10 and FIG. 12. Theseplots correspond to 0.0, 0.7 and 1.0 (full field) field of view and showhigh MTF values such as 0.4 or higher and more specifically of 0.5 orhigher at a broad range of frequencies (5 to 30 cycles per mm) almost upto full field of view.

Other performance criteria data is shown on FIGS. 8, 10, 11, and 13.These figures also show that the lens has an unusually high qualityperformance for a single lens element.

The lens shown in FIG. 1 focuses incoming on axial light rays down to aspot, therefore, the measure of quality for the lens will bepolychromatic point spread function. This plot shows the amount ofenergy at the center of the spot of light produced by the lens, as wellas the spread of this energy. The "spikier" this function, the smallerthe spread of energy, the better is the lens. This plot is shown in FIG.8. As we can see, this function is very "spikey" and thus the lensperformance is indeed very good.

The hybrid lens of the first embodiment accommodates at least a ±32°field of view. The MTF value of the lens far exceeds the requirement forsingle-use camera lenses (which generally have an area weighted averageMTF of about 0.3 in the range of 5 to 30 lines per mm) and provides awide margin of error for manufacturing tolerance.

In addition to mounting alignment error, errors can also arise in lensmanufacture. These errors will also effect adversely the performance ofthe lens element. However, because this lens has superior performance tobegin with, a relatively large margin of manufacturing errors than whatis typically allowed may be permissible. Thus, the price of the lens maybe further reduced.

Since the lens of FIG. 1 has a diffractive surface with diffractive,i.e., a blazed surface relief element, also called a "Blazed BleachedBinary", or blazed surface relief element, the lens efficiency at agiven λ is constant across the aperture. Therefore, this lens has highefficiency all the way from the center to the edge of the aperture.

With reference to FIG. 14, the diffractive/refractive hybrid lens 20 ofthe second embodiment is similar to the diffractive/refractive hybridlens of the first embodiment. The lens 20 is a convex-concave singletelement having a curved surface 1' and a diffractive zone pattern 3' onthe curved, underlying base surface 2' of the lens body which is thesurface opposite to the first curved surface 1'. Both surfaces 1' and 2'are perpendicular to the optical axis of the lens. The refractive lensis made from optically transmissive material having an index ofrefraction of at least 1.45 (it is 1.49 in this embodiment). FIG. 14shows the lens 20 and emphasizes its curved (substrate) base surface 2'which defines the refractive portion of the element as well as thediffractive zone pattern 3' which defines the diffractive portion of theelement. An annular ring 16' is part of the lens body and is merely forattachment and location in a camera barrel. The lens F-number is f/9 andit accommodates a field angle of ±35 degrees. The overall thickness ofthe lens is 1.6946 mm in this embodiment. The base radius of the curvedsurface 2' on which the diffractive grating of the lens 20 is formed, islocated is 10.9339 mm to a point along the optical axis on the right ofthat surface. The diffractive profile 3' of the diffractive portion ofthe lens element 20 has a radius of curvature that corresponds to aneffective focal length of 426.33 mm. Exemplary dimensions and spacingsare set forth in Table 2. The refractive index n.sub.λn is at the centerof visible range corresponds to nominal wavelength .sub.λn ≈587 nm andn.sub.λn is 1.4926. However, the unique feature of this design is thatthe lens, including the diffractive surface is not optimized for λ_(n)≈587 nm, but instead is optimized at the wavelength corresponding to theblaze wavelength λ_(b) =510 nm (corresponding to n_(b) =1.496) asdescribed below in the specification. The resulting lens is achromatizedfor the range of 440 to 625 nm.

                  TABLE 2                                                         ______________________________________                                        Sur-                                    V                                     face  Radius    Thickness Material                                                                              Index Number                                ______________________________________                                        1     7.0321    1.6946    Plexiglass                                                                            1.492 57.3                                  2     10.9339.sup.1                                                                           0         Plexiglass                                          3     NA.sup.2  0                 10,000                                                                              -3.5                                  Air             4.3444    Air     1.    0.0                                   Stop            25.6924                                                       Image -120.000.sup.3                                                          plane                                                                         ______________________________________                                         1 Base radius is 10.9339. This surface profile is an asphere, where AD =      -0.3862708E 3; AE = 0.4161633E 4; AF = -0.2147031 E5; and AG = 0.3952169E     7.                                                                            2 This surface profile is an asphere. The aspheric profile of the surface     as described by equation 12, and where AD = -0.1246612E 7; AE = 0.2106454     8; AF = -0.1467578E 9; and AG = 0.3613653E 11.                                3 Cylindrical shape.                                                     

The performance characteristics of this lens are provided in FIGS.23A-D.

The condition for achromatism of a refractive/diffractive doublet with apositive focal length is satisfied with a positive refractive portionand a weak positive diffractive portion due to the negative dispersionof the diffractive portion of the lens. Normally, a refractiveachromatic lens is composed of a negative and a positive focal lengthlens elements, since both element dispersions are positive. Considerdesign wavelengths λ_(f) =486 nm, λ_(n) =λ_(d) =588 nm, and λ_(c) =656nm.

The typical lens system is designed for a center wavelength λ_(n) of 588nm. However, taking an unusual approach, I optimized the lens system,including its diffraction profile for lens 20 for the central or blazewavelength of λ_(b) =510 nm, rather than at λ=587 nm as is done for lens10 of the first embodiment. I have discovered that by optimizing thediffraction surface 3' for λ_(b) =0.510 μm, that the efficiency of thelens system is increased substantially.

An index of refraction of any refractive material varies with wavelengthand can be calculated by use of the following formula: ##EQU9## wherevalues of A_(o), A₁, . . . , A₅ are determined by method of leastsquares using measured values of n at six or more wavelengths. TheSchott and Ohara optical glass catalogs provide these coefficients forevery optical glass. For example, an index of refraction for acrylic atany wavelength may be calculated using the above formula and knowingthat:

    A.sub.0 =2.1904237

    A.sub.1 =-0.2207493×10.sup.-2

    A.sub.2 =0.1158779×10.sup.-1

    A.sub.3 =0.2475242-10.sup.-3

    A.sub.4 =-0.3382604×10.sup.-6

    A.sub.5 =0.7357×10.sup.-8                            (15)

Thus, n(λ=0.5876μm)=1.491741.

In order to get 100% efficiency at any given wavelength, one needs toachieve a constructive interference effect. As stated above, in order todo that the rings or zones have to be spaced exactly one wavelength awayfrom the point F (FIG. 3). However, if one satisfies this condition ofone wavelength difference at same wavelength λ_(b), one will not be ableto satisfy this condition for other wavelengths, i.e. λ_(i) ≠λ_(b).Because the zones will not be spaced exactly one wavelength apart fromthe point F for wavelengths other than λ_(b), the efficiency of thegrating η(λ) will be lower for these wavelengths. The drop in thediffraction efficiency η depends on the detuning parameter α(λ) which inturn depends on the wavelength.

The detuning parameter α(λ) is characterized by ##EQU10## where we chosen_(b) =1.491741 and λ_(b) =0.5876 μm.

The detuning parameter is related to h_(max) where the h_(max) is themaximum height of the grooves across the lens surface as shown in FIG.4A.

The detuning parameter is related to h_(max) where the h_(max) is themaximum height of the grooves across the lens surface (FIG. 4) ##EQU11##where λ_(b) is the center or blaze wavelength of design and η(λ_(b)) isthe refractive index at this wavelength.

The detuning parameter α(λ) then can be characterized as h_(max).##EQU12## where h_(max) is a height of grooves for maximum diffractionefficiency at λ_(b) and h(λ) is the height the grooves would need to beif the diffraction efficiency was to be 100% at that wavelength. Thus,since the grooves can have only one height, the grating is detuned atwavelengths other than λ_(b).

As stated above, the diffraction efficiency η(λ) of the lens is afunction of wavelength and of the detuning parameter α(λ). Thediffraction efficiency η(λ) at any wavelength is calculated by:##EQU13##

The diffraction efficiency as a function of wavelength for the visibleregion, for the lens system optimized for λ_(b) =0.5876 μm is providedin FIG. 5. FIG. 5 shows that although the diffraction efficiency ishighest at 0.5876 μm and although the diffraction efficiency stayedfairly high (η<0.9) for wavelengths higher than 0.5876, it drops off to0.4 at 0.4 μm.

As a result, the total or overall efficiency K.sub.λblaze of the lensthroughout the wavelength region is about 0.86. This means that about86% of light on the visible spectrum will be diffracted by the lens'sdiffractive surface towards the focal plane. Total or overall efficiencyK.sub.λb is calculated as a normalized integral of the diffractionefficiency over the wavelength region--i.e. ##EQU14## where λ_(b) =0.587μm.

This is also the overall efficiency of the lens system 10 of the firstembodiment.

Since the wavelength region in photographic systems is limited by to thefilm spectral response at the red end and by the source spectralintensity at the blue end, I decided, for purposes of analysis andoptimization that, 0.4 μm≦λ≦0.670 μm. However, a different spectralrange may be used for the analysis and optimization (ex: 0.4 μm<λ<0.69μm or 0.49 μm<λ<0.65 μm or 0.44 μm to 0.62 μm as was done in the designof the second embodiment).

A fraction of the light (in the visible spectrum) which is undiffractedis thus calculated to be 1.0-0.8609=0.1391 or about 14%. I discoveredthat shifting the central wavelength towards the blue end of thespectrum increases the overall spectrum efficiency K of the system.

For acrylic, n(λ=0.540 μm)=1.49413. This value was calculated usingequation 14.

The diffraction efficiency η(λ) if the lens system at any particularwavelength is calculated from the equation 19 and is plotted on FIG. 15.As expected, the diffraction efficiency η is at its highest at the blazewavelength λ_(b) =0.54 μm and drops down as the wavelength eitherincreases towards 0.67 μm or decreases towards 0.4 μm. I then discoveredthat while the diffraction efficiency for higher wavelength regions dropto compare to that of the system having efficiency shown in FIG. 16, theefficiency drop in higher wavelength is more than compensated by theefficiency boost in lower wavelengths. For example, while at the red endof the spectrum (i.e. at λ=0.61 μm), the efficiency drop is from about95% to about 87%, the efficiency at the blue end of the spectrum (i.e.at λ=0.4 μm) increases from about 36% to about 60%.

The total efficiency K when diffraction profile is designed for λ_(b)i.e. λ blaze of 0.540 μm is 0.912962 (i.e. K₀.540 ≈0.912). Thus,changing the central wavelength to λ=0.540 μm would result in 37.4%decrease in undiffracted or scattered light. This is calculated below:##EQU15##

Undiffracted or scattered light produces an undesirable fog-likeappearance on the film. Since the optical film is more sensitive to bluelight rather than the red light, decreasing the amount of undiffracted(scattered) blue light rather than the red will further improve theimage quality.

FIG. 16 is the graph of the diffraction efficiency η of the opticalsystem where the whole lens and thus the diffraction profile isoptimized for λ_(blaze) =0.510 μm rather than the typical λ=0.588 μm. Itcan be seen from FIG. 16 that the diffraction efficiency in the redregion has dropped further from that of FIG. 15. However, the efficiencycurve is nicely balanced--there is a corresponding increase in thediffraction efficiency in the blue region. Calculating the totalefficiency K throughout the spectral region at λ_(blaze) =0.510 we get##EQU16##

Thus, the overall efficiency throughout the spectrum is about 92.2%. Thedecrease in undiffracted light from that of the lens optimized atλ_(blaze) =0.587 μm is calculated to be: ##EQU17##

Changing the design wavelength λ_(b) of the lens from 0.587 μm to 0.510μm resulted in unexpected decrease of the magnitude of undiffractedlight of 44%. That means, that 44% reduction in undesired, scatteredlight. Thus, changing the central wavelength for the diffractive portionof the lens--rather than designing the diffractive surface at thecentral wavelengths used in a refractive portion of the lens--i.e.provided a substantial improvement to the overall quality of the lensusing the central blaze wavelength of λ_(b) 0.510 μm for optimizing thelens design rather than λ_(b) =0.587 μm produces an optimum diffractionefficiency.

Alternatively, lenses can be designed with the traditional wavelengths(λ_(central) =0.587 μm, 4861 μm<λ<656 μm) and then the diffractivefeatures can be scaled to maximize the efficiency at 0.510 μm (or 0.530μm, 0.540 μm etc.) by using the curvatures and the aspheric coefficientsfrom the 587 nm design and scaling the curvatures and the aspherics by{n(510)/n(587)} which is approximately=0.510/0.587, i.e. the curvaturesand aspheric coefficients get weaker by the ratio of the wavelengths.

FIG. 25 depicts a simple camera such as a single-use camera 100 having acamera body 5, a diffractive/refractive hybrid lens 30, an aperture stopST, and a photographic film having a film surface 15. The camera 100includes other known structures or means for advancing the film and ashutter for enabling the exposure of the film to capture an image of thescene to be photographed. The film surface 15 is curved and iscylindrical. Its radius of curvature R may be 80<|R|<200 mm; i.e. theshape of the film at the image plane forms a segment of a cylindricalsurface. It is the concave surface 15 of the film segment that faces thelens.

With reference to FIG. 25, the diffractive/refractive hybrid lens 30 ofa third and more preferred embodiment is similar to the lenses of thefirst and second embodiments. The lens 30 is a convex-concave singleelement (or singlet) having from an object side a convex-curved surface1" and a Fresnel zone pattern 3" on a concave-curved surface 2" of thelens body which is the surface on the opposite side of the side havingthe first curved surface 1". Both surfaces 1" and 2" are perpendicularto the optical axis of the lens. The refractive lens is made fromoptically transmissive material having an index of refraction of 1.49.FIG. 25 shows the lens 30 and emphasizes its curved surface 2" whichdefines the refractive portion of the element as well as the Fresnelzone pattern 3" which defines the diffractive portion of the element.The lens F-number is f/9.19 and the lens accommodates a field angle of±34 degrees. The overall thickness t of the lens may be less than 4 mmand it is preferred that it be about 1.0-3.0 mm (millimeters). It is1.645 mm in this embodiment. The focal length of lens 30 is 32.75 mm andit accommodates the field angle of ±34°. The center of curvature of thebase radius of the curved surface (or substrate) 2" on which thediffractive surface 3" of the lens 30 is formed (shown in FIG. 3), islocated 10.7959 mm to a point along the optical axis on the right ofthat surface. Exemplary dimensions and spacings are set forth in Table 3below, the index n.sub.λn is at the center of range being measured atthe nominal wavelength λ_(n) 546.1 nm (or 0.546 μm) and n.sub.λn n is1.492.

                  TABLE 3                                                         ______________________________________                                        Surface Radius           Thickness Glass                                      ______________________________________                                        1       ASPHERE.sup.1    1.645     492574                                     2       10.79592         4.266                                                3       DIAPHRAGM                                                             4       IMAGE.sup.3                                                           LENS LENGTH 1.645                                                                           PTZ SUM (F/100) = 0.005455                                      ______________________________________                                        BF = 30.20;                                                                   Best Focus -0.61.                                                             ______________________________________                                        Image   Angular  Approx. Rel.  Max. Aper. With                                Height  Field    Illum. (Image)                                                                              No Vignetting                                  ______________________________________                                        7.09    12.49    0.87          F/9.2                                          15.05   24.98    0.57          F/9.2                                          1.65    33.65    0.27          F/13.6                                         ______________________________________                                        Pupil         Location Diameter                                               ______________________________________                                        Ent.          7.04     3.58                                                   Exit          4.27     2.83                                                   ______________________________________                                    

Notes: ##EQU18## The phase surface can also be modeled using the Sweattmodel. The Sweatt model is comprised of two surfaces separated by a highindex medium with zero axial thickness. Sweatt model of surface 2:

    ______________________________________                                        SURF. 2                                                                       C = .092627757 VERTEX RADIUS (1/C) = 10.7959                                  N(546.1) = 10707.7724 N(440) = 8627.5882 N(650) = 12744.8235                  SURF. 3                                                                       C = 0.092627544                                                                          D = -0.1470998E-07                                                                           F = -0.1555938E-09                                  k = 0.0000000                                                                            E = 0.2444375E-08                                                                            G = 0.3536176E-11                                   VERTEX RADIUS (1/C) = 10.79592481                                             ______________________________________                                    

3. Image surface is a cylinder with radius -90 mm. Long dimension ofimage format is measured along cylinder radius of curvature.

4. MTF data at best AWA (Area Weighted Average) compromise focus alongdiagonal of image format. Data is calculated using the phaserepresentation of surface 2. Diagonal of image format on cylinder wasapproximated by best fit sphere to corner with radius of -130 mm. Thatis, the performance analysis is along the best fit spherical radius tothe corner of the cylindrical film. For 35 mm film format, this radiusis -130 mm. The best fit radius will be different for other formats. Theradius of -130 mm was used for analysis purposes only. The actualcylinder radius of the film is -90 mm. It is measured along the longdimension of the film. Along the short dimension of the film, the radiusof curvature is infinity.

    ______________________________________                                        PERCENT FIELD (TANGENTIAL/SAGITTAL)                                           L/MM   AXIS    40%     60%    75%   90%   AWA                                 ______________________________________                                        5.     95.     93/94   92/91  88/84 80/76 91                                  12.5   84.     75/80   71/72  53/52 30/30 68                                  ______________________________________                                    

Wavelengths used are 650, 546.1, and 440 mm with respective weights of0.35, 0.5, and 0.15. The image format semi-diagonal is 20.58 (100%field).

"AWA" is area weighted average determined by the following obliquity andweights: Axis 0.15, 40% 0.26, 60% 0.33, 75% 0.18, 90% 0.08.

The performance characteristics of the lens are provided in FIGS. 26A-F.

In this preferred embodiment, the center wavelength of the diffractivefeatures and the central wavelength of the achromatization are shiftedto 546.1. The wavelength of 546.1 nm is very close to the peaksensitivity of the eye's photopic response curve. In addition, the shiftto this wavelength eliminates or minimizes a possibility of colorfringing which may occur under certain circumstances. It was discoveredthat the eye observes the least amount of color fringing from lensesmade with a central wavelength of 546.1 nm. Therefore, it is preferred,for the lens efficiency standpoint as well as for the "fringing" reasonsto achromatize single element hybrid lenses in single-use cameras orthose used in other simple cameras when the central wavelength is in the510 nm to 550 nm ranges. It is more preferable that the wavelength rangebe in the 535-550 range and it is even more preferable that 540<λ<550range because although the diffraction efficiency of the lens isslightly reduced when the central wavelength is shifted from λ=510 nm toλ≈546 nm, the reduction in the fringing effect is appreciable which inturn can result in higher quality photographs.

The second difference between the second embodiment and the thirdembodiment is that more emphasis was placed on eliminating astigmatismat the 0.4 field which resulted in changes to the aspheric coefficientson surfaces 1 and 2. This change results in decrease field curvature atthe 0.4 normalized field position. A bulge in the S+T curves near the0.4 field can cause some undesired astigmatism in images at positionsrelatively close to the optical axis. By placing more emphasis on theimage quality at the 0.4 field in the design process the field curvatureand astigmatism are decreased with the lens of the third embodiment. Asstated above, this was caused mostly by changes in the asphericcoefficients and also by the lens bending, i.e. changing radii ofcurvature of the lens element without affecting its focal length.

It was discovered that the diffractive/refractive lens allows larger NA(Numerical Aperture) for a given lens than a simple refractive lenselement.

The use of a hybrid refractive/diffractive lens allows the lens to workat a lower f/#, (i.e. higher NA) for a given level of image quality(spot size or MTF) than would be achievable with a simple refractiveelement due to the fact that the axial and lateral color aberrations arecorrected in the hybrid design whereas they are not corrected in thesimple single element refractive design. To correct for chromaticaberrations with refractive components a two-element design must beutilized which will increase cost and the need for precise alignment ofthe two lens elements relative to each other. The hybrid lens isfabricated with the diffractive surface already aligned to therefractive element since it is formed on the back of the refractivesection of the lens. Another reason for the ability of adiffractive/refractive lens to operate at higher NA's is thatdiffractive lenses do not have field curvature as an aberration.Therefore, a single lens composed of refractive and diffractivecomponents will also have less field curvature than a single elementrefractive lens, which will help to maintain the image quality over theextents of the photograph (better sharpness at larger field angles), andincreases the depth of focus budget for the lens overall. In the designsdisclosed herein, use of aspheric terms on the diffractive surface alsoenables the reduction of the monochroniate aberrations in conjunctionwith the aspheric terms on the front surface of the lens.

The disclosed hybrid lenses have superior performances in f/9 range orhigher and thus are especially suited for single-use or otherinexpensive cameras that have an F-number f/9 to f/11 because the lensperformance is superior to that of a regular single element refractivelandscape lens. When a diffractive/refractive lens can be used in asingle-use camera in conjunction with a curved image or film plane sothat the film is curved in a cylindrical form during the exposure by thelens element, the quality of printed pictures is further improved. Theradius of curvature R for the film should be in the -85 to -200 mmrange. However, it was found that when 85<|R|<95 mm with the concavesurface facing the lens, better results are obtained because this rangeof radii compensates for field curvature and distortion introduced bythe lens and contrary to common wisdom, no appreciable deterioration ofimage quality resulted from having the radius of curvature that issteeper than the industrial standard (which has |R|<100 mm and when Rusually is 120<|R|<190 mm.

Furthermore, among various advantages of a continuous blaze grating are:

1) higher diffraction efficiency;

2) elimination of alignment errors due to multiple step etching process;

3) elimination of wavefront errors due to etching process; and

4) the surface of the grating acts as an anti-reflection coating.

As previously described, a diffractive profile formed with "stepfunction" method, typically results in 4-16 discrete steps comprisingeach zone (FIG. 5A). Such diffractive profiles result in a smallerefficiency (even at the central wavelength). The efficiency of thediffractive profile formed by the "step function" method is described bythe following equation: ##EQU19## where p=2^(N), and

where N is the number of masks, p is the number of steps and m is thediffraction order. (In this case, m=1 since the first order is used andm=0 represents undiffractive light).

A diffraction profile manufactured by a "step function" method (i.e.binary method) and which is made with four mask levels, i.e. adiffraction profile comprised of four masks or 16 steps, is 98.7%efficient at λ_(o). A diffraction profile made of three masks and eightlevels is 85% efficient. On the other hand, with a continuous blazemethod, an efficiency is 100% at the central (i.e. blaze) wavelength.

A binary or a "step function" method also results in alignment errorswhich result in inaccuracies introduced in a diffractive surfaceprofile. The errors are introduced because the manufacturing processrequires that each mask level be aligned with respect to the other. Forexample, a single zone comprised of 16 steps is made with four masks.Each of the four masks has to be aligned with respect to the others.Although such masks are aligned to each other to within a fraction ofmicron, the alignment errors nevertheless cause decreased diffractionand introduce wavefront errors. At the present time, there is no knownmethod resulting in a perfect alignment of the masks. By cutting thewhole zone profile, one realizes an advantage of eliminating thesealignment errors and increases the diffraction efficiency and eliminatesthe wavefront errors introduced by inaccuracies in alignment as well.

Yet another advantage of the present invention is that the diffractivesurface of the lens also acts as an anti-reflection coating. I havediscovered that if the diamond turning marks on the diffractive surfaceare spaced by less than the wavelength of light i.e. d'<λ_(n) (see FIG.4B), light will not reflect or diffract from the pattern. This is avaluable property of the lens in that the amount of the unwanted orstray light is further reduced and thus the glare in the film may bereduced or eliminated. This kind of diffraction pattern can also beformed by other means, such as, lithography, but in the embodiments ofFIGS. 1 and 14, it is preferably formed by a continuous or linear blazemethod as described below.

In a preferred method of manufacturing these lenses, one calculates theexact phase for the diffractive wavefront and creates or specifies asurface profile to be cut with the diamond turning machine. The diamondturning machine may be used to cut the lens surface directly or,alternatively to cut a surface that will be used to manufacture a mold.The diamond turning machine is sampled at approximately 10 to 20 micronincrements and at the maxima and minima of each diffractive fringe.

In order to fabricate sharp corners at the peaks and valleys of thediffractive surface, it is preferred to utilize a sharp diamond pointtip (FIGS. 18, 20) as opposed to the traditional approach of using aradiused diamond tip (FIG. 19). Typically, diamonds used in diamondturning are fabricated with radii of 150 to 100 um. This radius helps toreduce surface roughness and increase lifetime, however, the radius onthe tool limits the sharpness of the corners. The decreased sharpness inthe corners leads to more scattering and more undiffracted light. Also,in many instances the zone spacings one is required to fabricate aresmaller than the radius of the diamond tips. Therefore, it would beimpossible to fabricate them with this type of a rounded diamond tip.

It is preferred to use a novel sharp diamond with a small flat (FIG. 18)(for example l≈1.0 to 0.5 um) on the end of the tip which is canted atan angle relative to the work piece to cut the diffractive surface (FIG.21). The tip may be canted more or less depending on a diffractivesurface profile at a particular location. It has been found that a sharpdiamond tip (FIG. 20) with no flat may be used with success, but thetool wear was much greater in this instance. The sharp diamond with theflat will cut plastic and metal (metal is cut in order to make a mold)to a high level precision with a relatively long tool life. Aconventional diamond turning equipment (such as ASG-2500-T produced byRank Pneumo [a division of Rank Taylor Hobson, Inc.]) was used. Thisequipment is shown schematically in FIG. 17 and comprises a translationstage TS which moves with the velocity V and a diamond tip T mounted onan arm A. The blank for the lens mold M to be used to manufacture thediffractive surface is mounted on a rotary spindle S which moves atrotary speed W. A cutting tool (ex. diamond) is moved radially along thesurface in program amounts to create a profile for the surface. Thesurface is cut in a series of cuts in a radial (spiral) fashion.However, a conventional diamond tip is replaced with a special diamondtip T (shown in FIG. 18) as described above. In addition, the machine isgenerated at different cutting rates than conventional cutting rates. Atypical conventional cutting speed for the rotary spindle is W-1000 RPM,where W is the rotational speed, the depth of the typically conventionalcut is 0.002 mm and the translational rate V is at the order of 5 mm perminute. Initially, at first pass, a rotational speed W of 450 RPM and atthe second pass a feed rate of 0.002 mm depth at the speed of 0.5mm/minute and at the second pass a feed rate of 0.0005 mm at 0.5mm/minute were used. However, it was found that cutting at a slowrotational speed W caused tool marks to appear at the surface. It wassubsequently found that the best results are achieved when therotational speed W is higher than the conventional rotational speed andwhere the transition stage velocity is lower and the cut shallower thanthe conventional transition stage velocity. More specifically, it ispreferred to use the rotational speed W of 2000 RPM. At the first(rough) pass, a cut at a depth of about 0.001 mm depth is made, the feedrate of 1 mm/minute. At the second (rough) pass, a cut at 0.0005 mmdepth at the speed of 0.5 mm/minute. One then progresses to a medium cutat the rates of 0.002 mm depths at the rate of 0.2 mm/minute feed rate.A finish step with a fine cut of 0.00002 mm depth at 0.05 mm/minute rateis then made. The initial blank to be cut is roughly of the desiredshape (i.e. diameter) that is required of the final surface.

This process allows us to have about 50 to 250 very shallow tool marksper zone which is very different from the 4 to 16 steps in zonesmanufactured by the prior art methods. Because the tool marks are manyand very shallow, due to the surface roughness characteristics, thediffractive surface acts as an anti-reflection coating and furtherimproves the efficiency of the optical system. The resultant surfaceroughness is less than λ/6 peak-to-valley (P to V) and λ/10 RMS and maybe as small as <λ/50 peak to valley or λ/100 RMS or even smaller. If thesurface is too rough, there is lots of scattering. If the surfacebecomes too smooth, the anti-reflection properties tend to worsen. Thesurface roughness can be measured by conventional electron microscopesor with other conventional measurement equipment. FIG. 22 shows finalsurface roughness characteristics of the preferred embodiments as seenthrough the atomic force microscope.

As one can see, the distance d between the tool marks is less than 1 μmand more specifically is about 0.5 μm. It is preferred that d be between0.2 to 0.6 μm.

Finally, the diffractive surface of the lens may be formed on a flat aswell as a curved substrate. Forming the diffraction pattern on a curvedsubstrate provides for an additional parameter (i.e., radius ofcurvature) to be used for aberration control.

The lens can be manufactured in a single step if the lens is molded.Single-step fabrication due to molding of a part is advantageous becauseit decreases cost, reduced number of fabrication steps and improvesalignment of parts optical axis. If a molding method of manufacturing isused, a mold having an appropriate surface profile has to bemanufactured first. A diffractive surface relief pattern is machined inmetal and it becomes a mold for the diffractive portion or diffractivesurface 3 of the lens. The mold for a refractive portion of the lens orsurface 1 is made by conventional techniques. The optical material isthen injected between the mold surfaces S1 and S2 along the sprue S andinto the gate G (FIG. 24). Once the optical material hardens, the mold(cavity block) is parted and the molded lens and the sprue is ejected.The final product (i.e. the lens 10, 20) is then broken off from therest of the plastic and is ready for final assembly.

From the foregoing description, it will be apparent that the inventionprovides a diffractive/refractive hybrid achromat designed for use as acamera objective such as inexpensive or single-use cameras. Utilizingthe is characteristics of a diffractive lens, such as a dispersion ofthe opposite sign of ordinary glasses, a diffractive/refractive hybridlens with an extremely good aberration control is obtained.

The use of a refractive/diffractive combination allows the use ofsmaller surface curvatures on each element which results in smalleraberrations, lower cost production and increased part tolerances. Theuse of a diffractive surface for the achromatizing element allows thefabrication of a single physical lens element which is achromatized andhighly corrected for aberrations. The diffractive surface can befabricated by etching or preferably machining the diffractive surfacerelief pattern onto one of the surfaces of a plastic injection mold.Alternately, the surface relief pattern can be formed via replication ofa mold pattern in epoxy on the surface of a glass lens. It is alsopossible to form a holographic optical element on the back surface ofthe lens if an emulsion (such as dichromated gelatin) is coated on thelens element and exposed to the correct laser interference pattern.

Finally, the hybrid lens according to the present invention, is notlimited to use in a single-use camera, but may be used in any simpleobjective lens camera or other image-capturing apparatus used to focuslight in the visible spectrum and especially those made of a single-lenscomponent or element. In addition, such a component or element may alsobe incorporated into a multiple-element objective system which requiresachromatization in the visible region. Although in the above threeembodiments the focal length is in 32 and 35 mm, the lens according tothe present invention is particularly suitable for use in single-use orother inexpensive cameras having 22 to 40 mm range and having anF-number f/5.6 or higher, and preferably 9<F<11 range.

Variations and modifications within the scope of the invention willundoubtedly suggest themselves to those skilled in the art. Accordingly,the foregoing description should be taken as illustrative and not in alimiting sense.

I claim:
 1. A consumer camera for use in visible applicationscomprising:(i) an optical system consisting of a singlerefractive/diffractive lens component having radii of curvature and adiffraction surface profile sufficient to achromatize said opticalsystem over the entire visible spectrum; (ii) a film for imaging anobject by said refractive/diffractive lens component; and (iii) a camerabody enclosing said film and a portion of said refractive/diffractivelens component, wherein said hybrid refractive/diffractive lenscomponent comprises a lens component body of optically transmissivematerial having an index of refraction at a wavelength approximately inthe center of the visible spectrum of at least 1.45, said lens componentbody having first and second surfaces on opposite sides thereof, atleast one of said surfaces being curved to provide a refractive portionhaving power and introduces chromatic aberration, said lens componenthaving a diffractive portion having power which substantiallyachromatizes said lens for said chromatic aberration of said refractiveportion over about 440 nm to about 650 nm, and 9<f#; where f# is anf-number of the optical system.
 2. A camera according to claim 1,wherein said film has a film portion which has a cylindrical shape.
 3. Acamera according to claim 2, wherein said film has a radius of curvatureR such that 85<R<200 mm.
 4. A camera according to claim 1 further havinga diaphragm or an aperture stop located between said lens component andsaid film.
 5. A camera for use in visible applications comprising:(i) anoptical system comprising a refractive/diffractive lens component havingradii of curvature and a diffraction surface profile sufficient toachromatize said optical system over visible spectrum; (ii) a film forimaging on an object by said refractive/diffractive lens component; and(iii) a camera body enclosing said film and a portion of saidrefractive/diffractive lens, wherein said film has a film portion whichhas a cylindrical shape.
 6. A camera according to claim 5, wherein saidfilm portion has a radius of curvature R such that 85<R<200 mm.
 7. Acamera according to claim 5, wherein said refractive/diffractive lenscomponent comprises a body of optically transmissive material having anindex of refraction at a wavelength approximately in the center of thevisible spectrum of at least 1.45, said body having first and secondsurfaces on opposite sides thereof, at least one of said surfaces beingcurved to provide a refractive portion having power and introduceschromatic aberation, said lens having a diffractive portion having powerwhich substantially achromatizes said lens for said chromatic aberrationof said refractive portion over about 440 mm to about 650 nm.
 8. Acamera for use in visible applications comprising:(i) an optical systemcomprising a refractive/diffractive lens component having radii ofcurvature and a diffraction surface profile sufficient to achromatizesaid optical system over visible spectrum; (ii) a film for imaging on anobject by said refractive/diffractive lens component; and (iii) a camerabody enclosing said film and a portion of said refractive/diffractivelens, wherein said lens component has the radii of curvature, thicknessand an aspheric profile so that,

    < f#<12

    26<f<34 mm

    |±30°|<w

where f# is a f# of the lens, f is the focal lens of the lens, and w isthe half field angle of the lens.
 9. A camera according to claim 8,wherein said refractive/diffractive lens components has an asphere on atleast of its surfaces.
 10. A camera according to claim 8, wherein saidrefractive/diffractive component has a blazed diffractive surface.
 11. Acamera according to claim 5, wherein said lens component has the radiiof curvature, thickness and an aspheric profile so that,

    9<f#<12

    26 <f<34 mm

    1+35° |<w

where f# is a f# of the lens, f is the focal lens of the lens, and w isthe half field angle of the lens.
 12. A camera according to claim 11,wherein

    9<f#<12

    32<f<34 mm

    1+±32° |<w.


13. A consumer camera for use in visible applications comprising:(i) anoptical system having an f-number greater than 9, said optical systemcomprising a single hybrid refractive/diffractive lens component havingradii of curvature and a diffraction surface profile sufficient toachromatize said optical system over the entire visible spectrum saidhybrid refractive/diffractive lens component comprises a body ofoptically transmissive material having an index of refraction at awavelength approximately in the center of the visible spectrum of atleast 1.45, said lens component body having first and second surfaces onopposite sides thereof, at least one of said surfaces being curved toprovide a refractive portion having power and which introduces chromaticaberration, said lens having a diffractive portion having power whichsubstantially achromatizes said lens component for said chromaticaberration of said refractive portion over about 440 nm to about 650 nm(ii) a film for imaging an object by said refractive/diffractive lenscomponent; and (iii) a camera body enclosing said film and a portion ofsaid refractive/diffractive lens component.
 14. A camera according toclaim 13 wherein said refractive portion has about 85-97% of the totalpower of the lens.
 15. A camera according to claim 13, wherein saiddiffractive portion has an Abbe ν-number, ν_(diff), and said ν_(diff) isnegative.
 16. A camera according to claim 13, wherein the diffractiveportion introduces a phase delay φ which can be described by thefollowing equation: ##EQU20## where λ_(o) is the wavelength in thecenter of said range, A₂, A₄, A₆, A₈, A₁₀, are phase terms, A₂ =1/(2f),where f is the focal length of the diffractive portion and A₄, A₆, A₈,and A₁₀ affect monochromatic aberration and depend upon the asphericcoefficients, AD, AE, AF and AG of the curved surface where Z is thesurface sag from a plane perpendicular to the optical axis tangent tothe curved surface where: ##EQU21## where C is the curvature of thecurved surface measured along the radial coordinate r from a point onthe optical axis.
 17. A camera according to claim 13, wherein the powerof the refractive portion φ_(ref), and the power of the diffractiveportion φ_(diff) add to provide the total power φ_(tot) of the lens, areproportional such that the longitudinal chromatic aberration isapproximately zero and are defined in accordance with the Abbe ν-number,ν_(diff) and ν_(ref) of the diffractive and refractive portions asfollows: ##EQU22##